The Bayer process is almost universally used to manufacture alumina. In this process raw bauxite ore is first heated with caustic soda solution at temperatures in the range of 140 to 250° C. This results in the dissolution (digestion) of most of the aluminum-bearing minerals, especially the alumina trihydrate gibbsite and alumina monohydrate boehmite, to give a supersaturated solution of sodium aluminate (pregnant liquor). Resulting concentrations of dissolved materials are very high, with sodium hydroxide concentrations being greater than 150 grams/liter and dissolved alumina being greater than 120 g/l. Any undissolved solids are then physically separated from the aluminate solution, and a polymeric flocculant is used to speed the removal of the fine solid particles. Residual suspended solids are removed by a filtration step. The filtered clear solution or liquor is cooled and seeded with alumina trihydrate to precipitate a portion of the dissolved alumina. After alumina precipitation, this depleted or spent liquor is reheated and reused to dissolve more fresh bauxite.
Bauxite ores used in the Bayer process also contain silica in various forms and amounts, depending on the source of the bauxite. The caustic used to dissolve the aluminum minerals also dissolves part or all of the silica content of the bauxite, especially the silica that is present in the form of aluminosilicate clays. The silica rapidly dissolves in the digestion step to form solutions that are supersaturated with respect to silica. This dissolved silicate reacts relatively slowly with the sodium aluminate in solution to form complex hydrated sodium aluminum silicates, generally designated “desilication products.” The principal desilication product is the species known as sodalite: 3(Na2O.Al2O3.2SiO2.2H2O)Na2X, where X can be CO3=2, 2Cl−, SO4=2, or 2AlO2−. Other related species such as cancrinite and noselite are also possible, so the more general term sodium aluminosilicate is preferred. All of these desilication products are of low solubility in the sodium aluminate liquor and largely precipitate out of solution, thereby removing undesirable silica from the solution.
The rate at which the desilication products precipitate out, however, is slow and even when a lengthy “predesilication” step is used, concentrations of dissolved silica remain well above equilibrium values. Some of this silica subsequently precipitates with the precipitated alumina and contaminates the alumina. Even after the alumina precipitation step, silica concentrations are still above equilibrium values in the so-called “spent liquor”, and because of the reduced aluminum concentrations, the silica becomes easier to precipitate out, in the form of sodalite and related minerals. An essential part of the Bayer process is to reheat this spent liquor so that it can be used to digest more bauxite ore. In the heat exchangers used to reheat the liquor, the higher temperatures increase the rate of aluminosilicate precipitation and as a result, aluminosilicate deposits as “scale” on the inside walls of the heat exchangers. The scale has low thermal conductivity compared to the steel of the walls and heat transfer is severely reduced as scale builds up. This reduced heat transfer caused by aluminosilicate scaling is sufficiently severe that the heat exchange units have to be taken out of service and cleaned frequently, as often as every one to two weeks.
Scaling that is related to silica can be minimized to some extent by a combination of blending bauxite ores with different silica contents, by optimizing the time and temperature of the digestion step, and by use of a separate desilication step. The situation is however complicated by the fact that silica in the solution or liquor is not necessarily proportional to the silica in the starting bauxite. Since the Bayer process is continuous, or cyclical, silica would continually increase if it were not removed from the system as aluminosilicate. Some silica is necessary to increase supersaturation to initiate precipitation of desilication products. Bayer liquors are always supersaturated with respect to silica and this excess silica can readily precipitate as aluminosilicate, especially onto the inside surfaces of heat exchangers.
There is considerable economic impact of aluminosilicate scale on alumina production. Cleaning of the heat exchangers with acid is itself a high maintenance cost. The acid cleaning also reduces the life of the heat exchangers, therefore adding cost due to frequent replacement of the heat exchangers. Moreover, the reduced heat exchanger efficiency caused by scaling leads to higher demand and cost for energy in the form of steam. The scaled pipes also result in decreased flow of liquor and potentially lost production. Altogether the costs directly due to scaling constitute a significant portion of the cost of producing alumina.
Scale build up has also been known to be a problem in boiler water systems and a number of treatments for reducing scale in boiler water systems have been proposed. In boiler water systems, pH is generally only 8 to 9 and dissolved salts are usually not present in concentrations more than about one to five grams/liter. Exemplary treatments for scale in boilers include siliconate polymers such as the copolymers of acrylic acid, 2-acrylamido-2-methylpropane sulfonic acid (AMPS), and 3-(trimethoxysilyl)propyl-methacrylate as disclosed by Mohnot (Journal of PPG Technology, 1 (1), (1995) 19-26). These polymers were reported to reduce the amount of silica gel adhering to the wall of polytetrafluoroethylene bottles in tests done with 645 ppm SiO2 at pH 8.3 and 100° C., i.e., conditions approximating those in a boiler. A Japanese patent application (Kurita Water Ind. Ltd., 11-090488 (1999)) also deals with adhesion of silica-type scale in cooling water or boiler water systems. The compositions disclosed are vinyl silanol/vinyl alcohol copolymers, which may also contain, e.g., allyl alcohol or styrene. Tests were done in water that contained 200 mg/l silica at pH 9.0 and temperatures of 45-75° C. Use of the subject compounds reportedly led to less silica scale compared to an acrylic acid-AMPS copolymer.
In boilers the pH is generally quite mild, only 8 to 9 and dissolved salts are usually not present in concentrations more than about one to five grams/liter. Additionally, scales formed in boiler water systems consist of primarily amorphous silica, although other scales such as calcium carbonate, calcium phosphate, etc., are possible. In contrast, the supersaturated solutions at high temperatures and high pH of essentially 14, make scaling problems much more serious and difficult to contend with in plants that carry out the Bayer process than in boilers. In addition, the concentrations of dissolved salts (i.e., sodium aluminate, sodium carbonate, sodium hydroxide, etc.) in the Bayer process are very high, such that total dissolved salt concentrations are greater than 200 grams/liter. It is not surprising, therefore, that the scales that form in the Bayer process are distinctly different from those that form in boilers and unlike boiler scales, all Bayer scales contain aluminum, which is expected because of the high concentrations of aluminum in the Bayer solutions or liquors. In particular, the aluminosilicate scales contain equal numbers of aluminum and silicon atoms.
Thus, although there have been treatments available for boiler scales, there has been limited success in obtaining methods and/or chemical additives that reduce or eliminate aluminosilicate scaling in the Bayer processing of alumina. The earliest attempts appear to be the use of a siloxane polymer (a silicon-oxygen polymer with ethyl and —ONa groups attached to the silicons), i.e.,
that reportedly reduced scaling during the heating of aluminate solutions (V. G. Kazakov, N. G. Potapov, and A. E. Bobrov, Tsvetnye Metally (1979) 43-44; V. G. Kazakov, N. G. Potapov, and A. E. Bobrov, Tsvetnye Metally (1979) 45-48). It was reported that at the relatively high concentrations of 50-100 mg/l, this polymer was effective in preventing decrease of the heat transfer coefficient of heat exchanger walls. Methods of altering the morphology of aluminosilicate scales have been disclosed using either amines and related materials (U.S. Pat. No. 5,314,626 (1994)) or polyamines or acrylate-amide polymers (U.S. Pat. No. 5,415,782 (1995)). While these materials were shown to modify the morphology of the aluminosilicate particles, there were no examples of reduction in the amount of scaling. Additionally, treatment concentrations required were quite high, in the range of 50 to 10,000 parts per million.
Hence, thus far no economically practical materials or process has been offered to solve the problem of aluminosilicate scaling in the Bayer process industry. There is, in fact, currently no way at all to eliminate aluminosilicate scaling in the Bayer process. Because of the severe problems caused by aluminosilicate scaling, it would be a great benefit to the industry to have a cost-effective treatment method that would reduce these problems and expenses.